In the processing of plastics, it is frequently necessary to remove relatively large quantities of gases or vapors, such as solvents, water and monomers from the material in the course of processing by the screw extruder. However, a limit is placed on the cross section and size of degassification channels in the screw extruder because of the inherent danger that turbulence in the emerging gas or vapor will entrain particles therein of the molten material being processed. Furthermore, this complicates the effective shaping of the extruder screws in the degassification region along the process path of the extruder including shearing and kneading elements thereof as well as the conveyor elements.
A degassing device for plastic processing screw extruders is known from FRG patent DE-PS No. 1554866 in which rollers are arranged in pairs parallel to one another in a degassification opening for returning plastic compositions escaping from the screw extruder. The rollers are opposite each other and form a roll nip therebetween and are rotatable in the direction of the degassification opening, the rollers being optionally provided with a fluted or profiled surface. By virtue of the relatively long degassing path along the extruder, considerable quantities of plastic particles pass through the free cross section of the roller nip which is necessary for degassification. The plastic particles collect on a headpiece of the housing and form solid residues which adhere to the gas discharge line. Such residues change in their physical structure and easily detach themselves and return into the processing material which leads to a reduction in quality.
Another degassification device for a screw extruder is known from European Patent EP-OS No. 01 14 999 in which a return device is arranged in the degassification channel parallel to the axis thereof, the return device being in the form of a conveyor screw extending throughout the cross section of the degassification channel and in communication with the screw chamber of the extruder. The cross section of the degassification channel is thereby limited to the cross section of the screw chamber of the extruder and does not permit the removal of gas and vapor in any amounts as great as desired and hence this device is only adapted to the processing of special products.
Another arrangement of two conveyor screws is shown in FRG patent DE-PS No. 915689 in which the two screws mesh with one another and strip each other in the degassification channel, the arrangement extending perpendicular to the extruder. The cross section of the arrangement is structurally limited. Namely, the diameter of the conveyor screws in the degassification channel is, at most, equal to the diameter of the extruder screw. Hence, with two conveyor screws which strip each other, the active length along the extruder screw for degassification corresponds, at most, to the axial spacing between the conveyor screws plus their diameters.